self: super:

let
  nixos = self.config.nixos or true;
  useUnstable = self.config.useUnstable or true;

  unstablePkgsExprs = if nixos
      then builtins.fetchTarball "https://channels.nixos.org/nixos-unstable/nixexprs.tar.xz"
      else builtins.fetchTarball "https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.xz";

  # Compiled nixpkgs expression eg expressions from a nix channel
  nixpkgsVersion = builtins.concatStringsSep "." [
    (builtins.readFile "${unstablePkgsExprs}/.version")
    (builtins.readFile "${unstablePkgsExprs}/.version-suffix")
  ];

  nixpkgsRevision = (builtins.readFile "${unstablePkgsExprs}/.git-revision");
in
import ./unstable-from-source.nix {
  unstableSource = unstablePkgsExprs;
} self super // {
  unstableVersion = self.lib.optionalString useUnstable nixpkgsVersion;
  unstableRevision = self.lib.optionalString useUnstable nixpkgsRevision;
}